1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming images by using a toner replenishing unit for supplying toner to either or both of a process cartridge and a developing cartridge in the image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer.
2. Related Background Art
Conventional electrophotographic image forming apparatuses have been adopting a process cartridge system in which a photosensitive body, charging or electrifying means, developing means, cleaning means, a toner container unit and the like are integrated into a cartridge. In this system, the cartridge is removably attached in the main body of the image forming apparatus concerned.
Such a cartridge system makes operation easier, and enables users themselves to do maintenance work on the process means. For these reasons, this type of cartridge system has been widely employed in the main bodies of the conventional image forming apparatuses.
Another type of cartridge configuration that can be used depending upon the life of the main process means has also been achieved, in which the process means is divided into long- and short-life groups and each group is integrated into a cartridge.
For example, a developing cartridge with a toner container or containers and developing means formed together therein, and a drum cartridge with an electrophotographic photosensitive body, charging means and cleaning means formed together therein have been adopted.
Recently, due to growth in demand for color electrophotographic image forming apparatuses capable of forming color images, expectations have been running for satisfaction of the following seven items: (a) low running cost, (b) small installation space, (c) low power consumption, (d) high quality, (e) high speed, (f) improved usability, and (g) ecology.
Conventional types of process cartridges and developing cartridges need to be replaced with new ones as soon as the cartridges run out of toner, which causes the following problems.
(1) In many cases, the above-mentioned cartridges are collected and recycled in cartridge-manufacturers' recycling systems or by private recycling dealers, but they may end up as waste.
It is therefore desirable to extend the life of the cartridges as long as possible, and hence to reduce the total amount of cartridge waste from environmental protection and resource saving standpoints. In other words, the life of the process means (such as an electrophotographic photosensitive body and a developing roller) and toner, both of which influence the life of the cartridge, need to be extended as long as possible.
In the event that the life of the process means is extended, an amount of toner corresponding to the life of the process means is supposed to be contained in the cartridge. The total weight of toner would increase in proportion to the life of the process means.
If the process means has a life span of up to 50,000 image copies, a required amount of toner will weigh 1.25 to 1.5 kg. If such a large amount of toner is contained in the cartridge, the total weight and volume will be necessarily increased, which runs the danger of reducing the operability.
(2) The main body of the image forming apparatus also needs a frame structure that can precisely support such a heavy cartridge, which results in an increase in the price of the entire apparatus.
(3) Further, in conventional toner replenishing type dual-component developing systems, a hopper for toner storage is provided in the main body of the image forming apparatus. In this case, toner is supplied from a toner replenishing container to the hopper, and to a developing device in this order.
In such a configuration, toner in the hopper can be used even if the toner replenishing container runs out of toner, which allows for a certain delay in exchanging cartridges.
The mechanism of the hopper part, however, increases the total number of parts, and hence the size of the cartridge, which also results in reducing the operability and increasing the total cost.
On the other hand, the time delay in exchanging cartridges makes it difficult not only to know the exact time to exchange cartridges, but also to measure the exact amount of residual toner in the toner replenishing container. This might cause trouble or image degradation in the process of image formation at the end of the life of toner, that is, as the toner replenishing container is running out of toner. Such a difference in image quality becomes visible especially in the formation of color images.
Since vivid color images cannot be formed even though there remains unused toner in the toner replenishing container, the time the cartridge needs replacing is brought forward, which makes it hard to effectively utilize resources despite the extended life cycle of the cartridge.